Gradient Ternary or Quaternary Multiple-Gate Transistor

ABSTRACT

An integrated circuit structure includes a semiconductor substrate; insulation regions over the semiconductor substrate; and an epitaxy region over the semiconductor substrate and having at least a portion in a space between the insulation regions. The epitaxy region includes a III-V compound semiconductor material. The epitaxy region also includes a lower portion and an upper portion over the lower portion. The lower portion and the semiconductor substrate have a first lattice mismatch. The upper portion and the semiconductor substrate have a second lattice mismatch different from the first lattice mismatch.

This application is a continuation application of U.S. patent application Ser. No. 12/616,068, filed Nov. 10, 2009, entitled “Gradient Ternary or Quaternary Multiple-Gate Transistor,” which claims the benefit of U.S. Provisional Application No. 61/182,550 filed on May 29, 2009, entitled “Gradient Ternary or Quaternary Multiple-Gate Transistor,” which applications are hereby incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATION

This application relates to commonly-assigned U.S. Provisional Patent Application Ser. No. 61/165,803, filed Apr. 1, 2009, and entitled “High-Mobility Multiple-Gate Transistor with Improved On-to-Off Current Ratio,” which application is hereby incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to integrated circuit devices, and more particularly to fin field-effect transistors (FinFETs) and methods for forming the same.

BACKGROUND

The speeds of metal-oxide-semiconductor (MOS) transistors are closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.

Compound semiconductor materials of group III and group V elements (referred to as III-V compound semiconductors hereinafter) are good candidates for forming NMOS devices due to their high electron mobility. In addition, germanium is a commonly known semiconductor material, and the electron mobility and hole mobility of germanium are greater than that of silicon, which is the most commonly used semiconductor material in the formation of integrated circuits. Hence, germanium is also an excellent material for forming integrated circuits. Therefore, III-V based and germanium-based transistors have been recently explored.

A challenge faced by the semiconductor industry, however, is that although the MOS transistors formed on III-V compound semiconductors or germanium have high drive currents, the leakage currents of these MOS transistors are also high. This is partially caused by the low bandgap and the high dielectric constants of the III-V compound semiconductors and germanium. For example, FIG. 1 illustrates the comparison of bandgaps and dielectric constants of germanium, commonly used III-V compound semiconductors, and other semiconductor materials such as group IV materials. FIG. 1 reveals that the bandgaps of germanium and some commonly used III-V compound semiconductors are small. Therefore, the respective MOS transistors suffer from high band-to-band leakage currents between their gates and source/drain regions. The high dielectric constants of these materials further worsen the leakage currents. As a result, the on/off current ratios (Ion/Ioff) of III-V based MOS transistors and germanium-based MOS transistors are relatively low.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an integrated circuit structure includes a semiconductor substrate; insulation regions over the semiconductor substrate; and an epitaxy region over the semiconductor substrate and having at least a portion in a space between the insulation regions. The epitaxy region includes a III-V compound semiconductor material. The epitaxy region also includes a lower portion and an upper portion over the lower portion. The lower portion and the semiconductor substrate have a first lattice mismatch. The upper portion and the semiconductor substrate have a second lattice mismatch different from the first lattice mismatch.

Other embodiments are also disclosed.

The advantageous features of the present invention include improved performance in MOS transistors including improved drive currents, reduced leakage currents, and high on-to-off current ratios.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates bandgaps and dielectric constants of semiconductor materials;

FIGS. 2 through 6 are cross-sectional views of intermediate stages in the manufacturing of a fin field-effect transistor (FinFET) in accordance with an embodiment of the present invention;

FIG. 7 illustrates a perspective view of the FinFET formed using the steps shown in FIGS. 2 through 6; and

FIGS. 8 and 9 are cross-sectional views of intermediate stages in the manufacturing of a FinFET in accordance with another embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the present invention are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

Novel fin field-effect transistors (FinFETs) and the method of forming the same are presented. The intermediate stages of manufacturing embodiments of the present invention are illustrated. The variations and the operation of the embodiments are discussed. Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements.

FIGS. 2 through 6 illustrate the cross-sectional views of intermediate stages in the manufacturing of a FinFET. Referring to FIG. 2, substrate 10 is provided. Substrate 10 may be a semiconductor formed of commonly used semiconductor materials such as silicon, germanium, SiC, SiGe, GaAs, or the like. Insulation regions such as shallow trench isolation (STI) regions 14 are formed in substrate 10. The formation process of STI regions 14 is known in the art, and hence is not repeated herein. The spacing S between neighboring insulation regions 14 may be small, for example, less than about 50 nm, and may even be less than about 30 nm, although spacing S may be greater. One skilled in the art will realize, however, that the dimensions recited throughout the description are merely examples, and will change if different formation technologies are used.

Next, as shown in FIG. 3, the portion of substrate 10 between opposite sidewalls of insulation regions 14 is recessed to form opening 18. Recessing depth D1 may be substantially equal to or less than thickness D2 of insulation regions 14.

In FIG. 4, semiconductor material 22 is epitaxially grown in opening 18. Semiconductor material 22 may include a material with a high electron mobility such as a III-V compound semiconductor material, which may include, but is not limited to, GaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlAs, AlP, GaP, combinations thereof, and multi-layers thereof. In an embodiment, semiconductor material 22 includes a gradient composition, with the compositions of lower portions gradually changed to the desirable composition of upper portions. Also, the bottom portion of semiconductor material 22 may have a lattice constant closer to the lattice constant of substrate 10, while the lattice constant mismatch between semiconductor material 22 and substrate 10 gradually increases from the bottom to the top of semiconductor material 22. In an exemplary embodiment, as schematically illustrated in FIG. 4, the bottom portion 22 ₁ is formed of GaAs, while substrate 10 is formed of silicon. The lattice constant of GaAs is greater than that of silicon, with the lattice constant mismatch between GaAs and silicon being about 4 percent. The top portion 22 ₃ of semiconductor material 22 may be formed of InGaAs with a composition between that of In_(0.5)Ga_(0.47)As and that of In_(0.7)Ga_(0.3)As. If formed of In_(0.5)Ga_(0.47)As, the lattice constant of top portion 22 ₃ is greater than that of silicon by about 8 percent. The middle portions of semiconductor material 22 have compositions between the composition of bottom portion 22 ₁ and the composition of top portion 22 ₃. Accordingly, the middle portions of semiconductor material 22 have lattice constants between the lattice constant of bottom portion 22 ₁ and the lattice constant of top portion 22 ₃. For example, in middle portion 22 ₂, the indium percentage increased from bottom to top, with a portion of an exemplary middle portion 22 ₂ having a composition In_(0.2)Ga_(0.8)As.

In alternative embodiments, bottom portion 22 ₁ is formed of germanium, and InGaAs is formed on germanium, with the percentage of indium in InGaAs gradually increasing from lower portions to upper portions, until the desirable composition such as In_(0.5)Ga_(0.47)As, In_(0.7)Ga_(0.3)As, or a composition therebetween is achieved in top portion 22 ₃.

Semiconductor material 22 may have a continuously changed composition, which may be achieved, for example, by continuously adjusting the flow rate of indium-containing gases such as trimethyl indium (TMIn), and/or gallium-containing gases such as trimethyl gallium (TMGa). The composition of semiconductor material 22 may also be layered, with substantially abrupt composition changes from layer to layer. The resulting composition in semiconductor material 22 (either continuously changed or layered) is referred to as a gradient composition.

In the resulting structure, top portion 22 ₃ of semiconductor material 22 is formed of a high-mobility III-V compound semiconductor material, which is at least a ternary material including three group III and group V elements. Alternatively, the high-mobility III-V compound semiconductor material further includes an additional group III and group V element to form a quaternary material such as InGaAlAs, InGaAlN, InPAsGa, and the like.

Referring to FIG. 5, the upper portions of insulation regions 14 are selectively etched, while bottom portions are left un-etched. As a result, semiconductor material 22 has a portion standing above the bottom portions of insulation regions 14, thus forming fin 24.

Next, as shown in FIG. 6, high-bandgap semiconductor layer 26 is epitaxially grown. In an embodiment, fin 24 has bandgap EgA, while high-bandgap semiconductor layer 26 has bandgap EgB greater than bandgap EgA. In an exemplary embodiment, bandgap EgB is greater than bandgap EgA by about 0.1 eV, although greater or lower bandgap differences may also apply. Conduction band EcA of fin 24 may also be lower than the conduction band EcB of high-bandgap semiconductor layer 26. In an exemplary embodiment, conduction band EcA is lower than conduction band EcB by about 0.1 eV, although greater or lower conduction band differences may also apply. The appropriate materials of high-bandgap semiconductor layer 26 may be selected by comparing the bandgaps of the available semiconductor materials with high electron mobilities, which may include, but are not limited to, silicon, germanium, GaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlAs, AlP, GaP, and the like. In an exemplary embodiment, high-bandgap semiconductor layer 26 comprises GaAs.

The structure shown in FIG. 5 may be used to form a fin field-effect transistor (FinFET). FIG. 6 further illustrates the formation of gate dielectric 30 and gate electrode 34. Gate dielectric 30 may be formed of commonly used dielectric materials such as silicon oxide, silicon nitride, oxynitrides, multi-layers thereof, and combinations thereof. Gate dielectric 30 may also be formed of high-k dielectric materials. The exemplary high-k materials may have k values greater than about 4.0, or even greater than about 7.0, and may include aluminum-containing dielectrics such as Al₂O₃, HfAlO, HfAlON, AlZrO, Hf-containing materials such as HfO₂, HfSiO_(x), HfAlO_(x), HfZrSiO_(x), HfSiON, and/or other materials such as LaAlO₃ and ZrO₂. Gate electrode 34 may be formed of doped polysilicon, metals, metal nitrides, metal silicides, and the like. The bottom ends of gate dielectric 30 and gate electrode 34 may contact the top surface of insulation regions 14.

After the formation of gate dielectric 30 and gate electrode 34, source and drain regions may be formed. A perspective view of resulting FinFET 100 (which may be an n-type FinFET) is shown in FIG. 7. FinFET 100 includes source region 44, drain region 46, and fin 24 therebetween. FinFET 100 may be an NFET, and source region 44 and drain region 46 are n-type regions doped with an n-type impurity, for example, by implantations. Alternatively, FinFET 100 may be a PFET, and source region 44 and drain region 46 are p-type regions doped with a p-type impurity.

In FinFET 100, with bandgap EgA of fin 24 being lower than bandgap EgB of high-bandgap semiconductor layer 26, fin 24 and the portion of high-bandgap semiconductor layer 26 on opposite sidewalls of fin 24 (refer to FIG. 6) form a quantum well. The quantum effect is partially affected by the thickness T of fin 24 (see FIG. 6), which thickness is equal to spacing S in FIG. 2. When a non-zero gate voltage is applied to gate electrode 34, due to the quantum confinement effect, electrons tend to flow through fin 24. Accordingly, the carrier mobility is high because of the low bandgap EgA, and hence the on-current Ion of the respective FinFET 100 is high. On the other hand, when FinFET 100 is turned off with the gate voltage being zero volts, electrons tend to flow through the high-bandgap semiconductor layers 26. Accordingly, the carrier mobility is low due to the high bandgap EgB, and hence the off-current (leakage current) Ioff of the respective FinFET 100 is low. FinFET 100 thus has a high on/off current ratio.

FIGS. 8 and 9 illustrate a second embodiment in accordance with another embodiment. Referring to FIG. 8, semiconductor substrate 10 is provided. Next, mask layer 50 is formed on semiconductor substrate 10. Mask layer 50 may comprise silicon oxide, which may be formed, for example, by thermally oxidizing a top layer of silicon substrate 10. Alternatively, mask layer 50 may also be formed by a deposition method such as one of the chemical vapor deposition methods. The materials of mask layer 50 may include, but are not limited to, silicon nitride, silicon oxynitride, and the like. Mask layer 50 may have a thickness between about 200 nm and about 450 nm.

In FIG. 9, mask layer 50 is patterned, for example by etching, and hence opening 18 are generated. Semiconductor substrate 10 is exposed through opening 18. In this embodiment, the remaining portions of mask layer 50 act as insulation region 14 and have substantially the same function as insulation region 14 as shown in FIGS. 2-6. The remaining steps of this embodiment are substantially the same as the steps illustrated in FIGS. 3-6, and hence are not repeated herein.

The embodiments of the present invention provide low-cost processes for growing high-mobility and low defect III-V compound semiconductor materials by gradually adjusting compositions of the III-V compound semiconductor materials. The transistors formed on the III-V compound semiconductor materials may have increased Ion/Ioff ratios due to the use of small bandgap channels and large bandgap leakage paths.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the invention. 

What is claimed is:
 1. An integrated circuit structure comprising: a semiconductor substrate having a first lattice constant; insulation regions on the semiconductor substrate and having sidewalls facing each other; an epitaxy region comprising a III-V compound semiconductor material over the semiconductor substrate, the epitaxy regions having sidewalls adjoining the sidewalls of the insulation regions, wherein the epitaxy region further comprises: a fin protruding above the insulation regions and having a second lattice constant different from the first lattice constant; and a gradient epitaxy region between the fin and the semiconductor substrate, wherein the gradient epitaxy region has a third lattice constant between the first lattice constant and the second lattice constant, and wherein sidewalls of the fin are substantially vertically aligned to sidewalls of the gradient epitaxy region; and a high-bandgap semiconductor layer on a top surface and the sidewalls of the fin, wherein the high-bandgap semiconductor layer has a bandgap greater than a bandgap of the fin.
 2. The integrated circuit structure of claim 1 further comprising: a gate dielectric on the high-bandgap semiconductor layer; a gate electrode over the gate dielectric; and a source region and a drain region on opposite ends of the fin, wherein each of the fin and the high-bandgap semiconductor layer extends from the source region to the drain region.
 3. The integrated circuit structure of claim 1, wherein a bottom surface of the high-bandgap semiconductor layer contacts the insulation regions.
 4. The integrated circuit structure of claim 1, wherein the fin is formed of a ternary III-V compound semiconductor material.
 5. The integrated circuit structure of claim 1, wherein the fin is formed of a quaternary III-V compound semiconductor material.
 6. The integrated circuit structure of claim 1, wherein the gradient epitaxy region has a gradient lattice constant, with lower portions of the gradient epitaxy region having smaller lattice constants than lattice constants of upper portions of the gradient epitaxy region.
 7. The integrated circuit structure of claim 1, further comprising a III-V compound semiconductor material layer interposed between the gradient epitaxy region and the semiconductor substrate, the III-V compound semiconductor material layer having a fourth lattice constant, the fourth lattice constant being between the first lattice constant and a lattice constant of the gradient epitaxy region at an interface between the III-V compound semiconductor material layer and the gradient epitaxy region.
 8. The integrated circuit structure of claim 7, wherein the III-V compound semiconductor material layer comprises GaAs.
 9. An integrated circuit structure comprising: a semiconductor substrate having a first lattice constant; a first epitaxy region above the semiconductor substrate, the first epitaxy region having a gradient composition such that a concentration of a first element increases as the first epitaxy region extends away from the semiconductor substrate, a first end of the first epitaxy region closest to the semiconductor substrate having a lattice constant closer to the first lattice constant than a lattice constant of a second end of the first epitaxy region, the second end being opposite the first end; insulation regions on opposing sides of the first epitaxy region, the insulation regions extending above a lower surface of the first epitaxy region, the first epitaxy region extending above the insulation regions forming a fin; and a high-bandgap semiconductor layer on a top surface and sidewalls of the fin, wherein the high-bandgap semiconductor layer has a bandgap greater than a bandgap of the first epitaxy region.
 10. The integrated circuit structure of claim 9, further comprising a second epitaxy region interposed between the first epitaxy region and the semiconductor substrate.
 11. The integrated circuit structure of claim 10, wherein the second epitaxy region comprises germanium.
 12. The integrated circuit structure of claim 11, wherein the first epitaxy region comprises InGaAs, wherein a percentage of indium increases as the first epitaxy region extends away from the semiconductor substrate.
 13. The integrated circuit structure of claim 11, wherein the high-bandgap semiconductor layer is GaAs.
 14. The integrated circuit structure of claim 9, wherein the first epitaxy region contacts the semiconductor substrate.
 15. An integrated circuit structure comprising: a semiconductor substrate having a first lattice constant; a first epitaxy layer over the semiconductor substrate, the first epitaxy layer having a second lattice constant; a second epitaxy layer over the first epitaxy layer, the second epitaxy layer having a third lattice constant, the second epitaxy layer having a gradient composition, the second lattice constant being between the first lattice constant and the third lattice constant; and isolation regions on opposing sides of the first epitaxy layer, an upper surface of the isolation regions extending higher than an interface between the first epitaxy layer and the second epitaxy layer.
 16. The integrated circuit structure of claim 15, wherein the first epitaxy layer is GaAs or Ge.
 17. The integrated circuit structure of claim 16, wherein the second epitaxy layer is InGaAs, wherein a concentration of indium increases as the second epitaxy layer extends away from the semiconductor substrate.
 18. The integrated circuit structure of claim 15, further comprising a high-bandgap semiconductor layer on a top surface and sidewalls of the second epitaxy layer, wherein the high-bandgap semiconductor layer has a bandgap greater than a bandgap of the second epitaxy layer.
 19. The integrated circuit structure of claim 15, wherein a lattice constant of the second epitaxy layer distal from the semiconductor substrate is 8% greater than a lattice constant of the semiconductor substrate.
 20. The integrated circuit structure of claim 19, wherein the second lattice constant is 4% greater than the first lattice constant. 